Measured Reciprocity Failure of Ilford Delta 100

I recently purchased a Gossen LunaPro SBC light meter and was given the LAB darkroom attachment by a forum member. I used this to measure the reciprocity failure of Delta 100 in 120 format. I exposed a roll from a uniformly illuminated white subject (my enlarging easel illuminated only by the enlarger with no colour filtration) with shutter speeds in 1-stop intervals from 1/2 second to 512 seconds (8 minutes 32 seconds). The exposures were chosen to give roughly Zone V (middle grey) at ISO 100 in the absence of any reciprocity failure. I then measured the densities of the resulting negatives using the LunaPro LAB attachment. The additional exposure (reciprocity failure correction) required to obtain a Zone V exposure was then calculated from the HD curve in the Delta 100 datasheet, and plotted on a logarithmic axes.

I found that no reciprocity failure correction is required for measured exposures of up to 16 seconds (by "measured exposure" I mean the exposure indicated by a light meter). For measured exposures longer than 16 seconds, and additional 1/3 stop of exposure is required for every stop (doubling of exposure time) by which the measured exposure exceeds 16 seconds. So for example if the measured exposure is 30 seconds, which is approx. 1 stop more exposure than 16 seconds, then an additional 1/3 stop would be required to correct for reciprocity failure, giving an actual exposure of about 38 seconds.

Note that this is to correct the Zone V densities. Since negative film has much more latitude for over-exposure than for under-exposure, it makes more sense to me to correct the Zone III densities where shadow detail is typically placed, in order to retain texture, and allow over-exposure of the highlights rather than under-exposure of the shadows (this can be corrected during development by reducing the development time, or in printing by using a softer paper grade). To do this, simply change the heuristic to say no additional exposure is required for 4 seconds, and then add 1/3 stop additional exposure for each stop of exposure beyond 4 seconds. In this case a 30 second exposure is 3 stops longer than a 4-second exposure, so requires an additional 1 stop of exposure, resulting in a 60 second actual exposure.

These characteristics are quite different from the curve published in the Delta 100 data sheet which suggests a 155 second adjusted exposure for a measured exposure of 30 seconds.